1. Field of the Invention
The present invention relates to a fluid or hydraulic pressure sensor suitable for sensing high fluid pressure and, more particularly, to a sensor for sensing pressure utilizing magnetostrictive effect generated by an amorphous magnetic alloy.
2. Description of the Prior Art
A fluid pressure sensor using a magnetostrictive effect generated by an amorphous magnetic alloy has been already proposed, and it is often used for measuring oil pressure. FIG. 1 shows a cross-sectional view of a conventional fluid sensor in which 12 indicates a magnetostrictive disc plate made of an amorphous alloy and having a thickness of about several tens of micrometers; 13 is a cylindrical body made of a soft magnetic material; 14 is a coil disposed on an annular groove 13a formed in cylindrical body 13; 15 is a housing in which the above-described members are provided; 16 is an O-ring which prevents oil from flowing out along the surface of magnetostrictive disc plate 12; 17 is an inlet for introducing oil; 18 is a through hole for the oil passage; and 19 is a circuit coupled with coil 14 for measuring inductance. Cylindrical body 13 and magnetostrictive disc plate 12 define a magnetic circuit for the magnetic flux generated by coil 14.
When the fluid pressure P is introduced from inlet 17 through holes 18 to amorphous alloy magnetostrictive disc plate 12, plate 12 partially distorts, such as to produce dimples shown by a broken line, thereby changing the magnetic permeability. As a result, the number of lines of force, representing the magnetic field as generated by coil 14, decreases. Accordingly, the inductance of the magnetic circuit, as measured by coil 14, varies relatively to the pressure applied to magnetostrictive disc plate 12.
According to the prior art pressure sensor as described above, since magnetostrictive disc plate 12 is very thin, it may be deteriorated after a long period of use or when it receives a very high pressure such that magnetostrictive disc plate 12 loses it elasticity to stay in the distorted shape even after the pressure is removed. Also, since magnetostrictive disc plate 12 is exposed to the oil coming from holes 18, magnetostrictive disc plate 12 may corrode after some period of use.